High power, low-noise erbium doped fiber amplifiers (EDFA) are required in dense wavelength division multiplexed communication links. Current 32 and 40 channel links are estimated to require EDFAs having output powers in excess of 22 dBm and uniform gain over a 32 nm bandwidth. These EDFAs require high power single mode pump lasers to efficiently pump the single mode erbium fiber.
Cost effective, high-power, single transverse-mode, EDFA pump sources are not found in the prior art. Single mode pump diode technology is limited to kink free fiber coupled output powers in the 175 mW regime. Although these sources can be wavelength division multiplexed together to achieve required pump powers, this approach is both costly and bulky. Current cost estimates indicate $17,600 for a multiplexed 450 mW pump source.
A 500 mW, Ytterbium (Yb) fiber laser requires approximately 1.5 W of 917 nm laser diode pump power at a receiving facet. This level of pump power can be reliably achieved using a broad area, multi-transverse-mode diode laser having a 100 .mu.m by 5 tm stripe. Since basic physics precludes efficient coupling of the output from the broad area diode into a single mode fiber, an alternate pumping scheme must be used.
Efficient pumping of rare earth doped fiber lasers using broad area laser diodes has previously been achieved using a double clad fiber. Known dual clad amplifier or laser fibers includes an inner rare earth doped single mode core and an inner cladding which support the lasing or signal wavelength. The inner cladding to the lasing core is designed to be a multimode core for the pump light. The inner cladding is followed by a final, outer cladding to confine the pump radiation.
An example of a known fiber laser is disclosed in U.S. Pat. No. 4.815,079 assigned to the Polaroid Corporation. The '079 fiber laser is characterized by a single mode core disposed within a relatively large multimode inner cladding. The inner cladding is surrounded by a further layer to prevent radiation from propagating out of the cladding.
Pump core dimensions of the commercially available Polaroid double clad fiber laser are 4.3 .mu.m for the circular inner core diameter, 170 mm.times.330 mm, for a rectangular inner cladding and an outer fiber diameter of 630 .mu.m. This fiber has been used to make high power Yb fiber lasers which operate at 1.010 .mu.m.